Depolarization potentials created during a muscle fiber contraction generate an electrical field gradient that propagates in a direction along the fibers throughout the volume conductor comprised of the muscle, the surrounding tissue, and skin layers. Electrodes placed on the skins surface allow for the non-invasive detection of this electrical field gradient providing the temporal summation of the propagating depolarization potentials of the active muscle fibers in the underlying vicinity of the electrode. The resulting voltage on the skin is the sEMG signal.
In order to measure this voltage, an electrolytic interface is formed between the electrolytes in the subcutaneous tissue and the ohmic electrically conductive surface of the electrode contact attached to the skins surface. The primary electrical conduit between the subcutaneous volume conductor and the skins surface is established via the sweat ducts which pass through the non-conductive stratum corneum so that sweat and moisture from the underlying sweat glands are deposited onto the skins surface completing the electrolytic interface.
The electrolytic interface consists of disassociated ions from the electrolyte forming a layer on the conductive electrode contact surface (Nernst polarization or contact half-cell potential). Depending on the chemical composition, concentration of the electrolytes on the skin, and the composition of the electrode contact metal, the half-cell potentials can range in amplitude up to several hundred millivolts.
Signal potentials emanating from the muscle in the underlying tissue are conveyed via ionic transport through the electrolyte to the exposed conductive contact surface of the electrode.
The signal amplitude may be several orders of magnitude smaller than the half-cell potential and ranges from 10 microvolts to 5 millivolts. The resultant voltage sensed by the electrode contact is therefore the electrical summation of the signal potential and contact half-cell potential.
When the electrolytic skin interface of the electrode is mechanically disturbed due to relative movement or pressure changes between the tissue and conductive surface of the electrode, the effective concentration of the electrolytes can be altered so that the resultant half-cell potential amplitude is modulated by the mechanical disturbance. The modulation of hall-cell potential is termed “movement artifact” and typically arises from rapid body movements, or objects or clothing coming into contact with the sensor case housing the electrodes.
Movement artifact can be particularly problematic as the change in half-cell potential can exhibit large (>50 mV) voltage deviations which overwhelm the amplitude of the sEMG signal.
An additional source of movement artifact is due to the triboelectric charge that can accumulate on the non-conducting stratum corneum as a result of walking on carpet or contact with certain fabrics under low humidity conditions. This effect can be especially problematic when the electrolytic skin interface exhibits high impedance resulting from the lack of suitable moisture between the electrode contact and the skin. This impedance can reach tens of megohms for contacts with an area of 1 mm squared placed on unprepared skin.
Common teaching dictates that the configuration of a sensor designed to detect sEMG signals consists of two electrode contacts placed on the skin over the muscle and oriented in a direction parallel to the muscle fibers. A third “reference” contact is preferentially located at an electrically inactive location on the body. Characteristically, disposable sEMG sensors preferentially designed for clinical use consist of two electrodes filled with skin impedance reducing electrolytic gel or formed from hydrophilic gel; one for each signal input placed singularly, or in pairs, mounted on a flexible non-conductive pad adhered to the skin over the muscle. In some sensors the two signal and reference contacts are placed on the same insulating pad in the form of an equilateral triangle.
The electrodes are attached by snaps or spring loaded clips and connected to remote electronic circuitry via individual lead wires. The preferred recording configuration is the single differential configuration where the voltage at each signal input contact is measured with respect the third reference contact and subtracted using a differential pre-amplifier circuit. In this way, any voltages common to both electrodes such as half-cell potentials and line interference effectively subtract to zero for an ideal amplifier.
However, in some disposable, single use, sEMG electrode designs, disturbances to the electrode interface induced from contact forces applied directly to the interface or induced from shear forces applied to the interface through the snap leads and interconnection cable movement are likely to cause an unequal localized disruption of the electrolyte junction half-cell potential of each signal electrode contact. This unequal change in half-cell potentials can not be removed by differential subtraction and as a result generates a movement artifact signal.
Additionally, a foam-backed disposable sensor interface may be susceptible to triboelectric charges that can accumulate on the non-conducting stratum corneum as a result of walking on carpet or contact with certain fabrics under low humidity conditions. The problems of movement artifact and sensitivity to electro-static fields are especially severe when the sensor is placed under clothing garments.
As further background, some reusable tethered and wireless sensor designs address the issue of movement and electrostatic artifact suppression by utilizing an enclosed, shielded case incorporating integrated preamplifier circuitry with signal and reference electrode contacts secured to the bottom. These sensors are placed directly on the muscle of interest and eliminate signal lead cable artifacts, however, their larger sensor dimensions and mass may preclude their placement on multiple, smaller adjacent muscles. Compared to disposable, single use configurations, reusable sensors also incur the additional steps of sanitizing procedures for repeated use in the clinical environment.
All of the aforementioned electrode contact and sensor configurations described as prior art offer only a limited set of solutions for detecting high fidelity sEMG signals in applications involving dynamic contractions. Disposable, single use configurations are convenient to apply and provide a hygienic implementation for clinical applications. However, their susceptibility to electrode and cable induced movement artifact precludes their use in vigorous applications and during conditions where electro-static fields may be generated such as sensor placement under an individual's clothing. Reusable, encased sensor designs with active electronics can suppress artifacts by eliminating the signal lead cables and by stabilizing the electrodes, yet lack compliance to fit to the contours of smaller underlying muscles which may encumber muscle movement during large flexions and extensions during vigorous dynamic activities.
The single differential recording configuration is most commonly used in both disposable and re-useable sensor designs. While suitable for the general evaluation of muscle activity, the single differential configuration is susceptible to the signal crosstalk interference generated from nearby active muscles. This precludes its use in applications requiring measurement of concurrent isolated muscle activity from multiple, adjacently located muscle groups. These applications are diverse and can range from sports and ergonomic activities to the clinical evaluation of patients with gait problems, Parkinson's disease, and other motor disorders. The double differential sensor configuration offers a potential solution for minimizing the effect of signal crosstalk in these applications. The technique uses an additional differential amplifier to subtract out the predominately in-phase crosstalk signal components present in both signal outputs of two differentially amplified contact pairs. However, the requirement of additional electrode contacts, increased sensor area, and more complex electronic circuitry has precluded its general acceptance.
It would be an improvement in the art to provide a disposable adhesive sensor configuration with integrated lead cable which can mold to the contours of the skin, which can comply with skin movement, and which can suitably isolate and detect muscle signals while suppressing movement artifacts and the effects of electro-static fields. When configured into a multi-sensor array, these low-profile, low-mass sensors would be applicable to the unencumbered measurement of muscle activity from multiple adjacent muscles, especially from smaller muscles such as those located in the face, neck, and hand. It could be used in conjunction with both existing tethered and wireless sensor technologies utilizing single and double differential recording configurations.